VI. SOIL ALGÆ 99 B. MURIEL BRISTOL, D.Sc., Algologist. VII. SOIL FUNGI--THE OCCURRENCE OF FUNGI IN THE SOIL 118 W. B. BRIERLEY, D.Sc., Head of the Department of Mycology. VIII. SOIL FUNGI--THE LIFE OF FUNGI IN THE SOIL 131 W. B. BRIERLEY, D.Sc., Head of the Department of Mycology. IX. THE INVERTEBRATE FAUNA OF THE SOIL (OTHER THAN PROTOZOA) 147 A. D. IMMS, D.Sc., Head of the Department of Entomology. X. THE CHEMICAL ACTIVITIES OF THE SOIL POPULATION AND THEIR RELATION TO THE GROWING PLANT 164 Sir E. JOHN RUSSELL, F.R.S., Director. INDEX 181 CHAPTER I. THE DEVELOPMENT OF THE IDEA OF A SOIL POPULATION. From the earliest times agriculturists have been familiar with the idea that decomposition of vegetable and animal matter takes place in the soil, and that the process is intimately connected with soil fertility. By the middle of the nineteenth century three different ways were known in which the decomposition occurred. One had been since early times specially associated with soil fertility, in that it gave rise to humus, the black sticky substance in farmyard manure or in soil--which was supposed up to 1840 to be the special food of plants. No good account of the process or of the conditions in which it occurred is, however, given by the older writers. A second resulted in the formation of nitrates. This process became known as nitrification: it was described by Georgius Agricola (1494-1555) in his book “De Re Metallica,” and it was of great importance in the seventeenth and eighteenth centuries, because it was used for the manufacture of gunpowder in the great wars of that period. The conditions for the making of successful nitre beds were so thoroughly investigated that little fresh knowledge was added to that of 1770[A] until quite recently. This process, however, was not usually associated with soil fertility, although both Glauber (1656) and Mayow (1674) had insisted on the connection. [A] See the remarkable collection of papers entitled “Instructions sur l’établissement des nitrières,” publié par les Régisseurs-généraux des Poudres et Salpêtre. Paris, 1777.